1. Field of the Invention
The present invention relates to a polyvinyl alcohol resin composition and films. More specifically, the present invention relates to a polyvinyl alcohol (hereinafter abbreviated as “PVA”) resin composition which is free from emanation of odor in a melt-forming process and excellent in transparency, and is capable of providing a product free from residual odor and a laminate film having higher interlayer adhesive strength. The present invention further relates to films produced by using such a PVA resin composition.
2. Description of the Related Art
A PVA resin generally has a fusion starting temperature that is very close to its thermal decomposition temperature. This makes it difficult to produce a product directly from the PVA resin by a melt-forming method. A conceivable approach to improvement of thermal stability in the melt forming process is to reduce the fusion starting temperature of the PVA resin by addition of a plasticizer such as glycerol or polyethylene glycol.
For example, JP-A-HEI9 (1997)-272719 proposes a resin composition prepared by blending a compound containing 2 to 4 moles of an alkylene oxide per 1 mole of a polyvalent (tri- or more valent) alcohol as a plasticizer with a PVA resin having a polymerization degree greater than 500 and not greater than 5000, and discloses that the resin composition is excellent in thermal stability in the melt-forming process and a product melt-formed from the resin composition is excellent in toughness and water solubility and substantially free from migration of the plasticizer.
The resin composition is effective to some extent for solving the aforementioned problem, but the product melt-formed from the resin composition suffers from fish eyes and gelation, requiring improvement.
On the other hand, JP-A-2001-302868 proposes a resin composition prepared by blending a compound containing 1 to 4 moles of an alkylene oxide in average per 1 mole of a polyvalent (tri- or more valent) alcohol as a plasticizer with a PVA resin having a viscosity average polymerization degree of 200 to 500, and discloses that the resin composition is excellent in melt spinnability and melt formability and is capable of providing various types of melt-formed products which are excellent in mechanical properties such as shock resistance and strength and substantially free from fish eyes, gelation and surface migration of the plasticizer and have a smooth appearance.
In examples of the invention disclosed in JP-A-2001-302868, a non-modified and partially saponified PVA resin having a saponification degree of 80 mol %, and PVA resins modified with an α-olefin such as ethylene, octyl vinyl ether or 7-hydroxy-1-octene are used. These PVA resins each have a lower crystallinity, and the use of any of these PVA resins reduces the melt viscosity of the resin composition, thereby improving the formability of the resin composition.
Even with the use of the plasticizer and any of the lower crystallinity PVA resins, a temperature not lower than 200° C. is required for the melt-forming of the resulting resin composition. This leads to thermal decomposition and/or evaporation of the plasticizer and other additives, detachment of an acetyl group from the partially saponified PVA resin, and detachment, thermal decomposition and/or evaporation of a modification group from the modified PVA resin, thereby causing problems associated with odor and environmental pollution. Where the plasticizer is less compatible with the PVA resin, phase separation is liable to occur in the melt-forming process, thereby reducing the transparency. Further, where the PVA resin composition is used for production of a laminate film, the surface migration of the plasticizer impairs the interlayer adhesive strength.
In general, the major main-chain bonds of the PVA resin are 1,3-glycol bonds. It is known that a PVA resin including a greater proportion of main-chain 1,2-glycol bonds than an ordinary level (about 1.8 mol %) can be prepared for melt-forming thereof by increasing the proportion of head-to-head bonds or tail-to-tail bonds through polymerization of vinyl acetate at a higher polymerization temperature (JP-A-2001-181405).
However, the main-chain 1,2-glycol bonds are less effective for the reduction of the crystallinity of the PVA resin than side-chain 1,2-diol structural unit. Further, hydroxyl groups of the PVA resin are all secondary hydroxyl groups as in ordinary PVA resins. Therefore, strong hydrogen bonds and intermolecular cohesive force attributable to primary hydroxyl groups cannot be expected from this PVA resin.
It is also known that a PVA resin having a hydroxyalkyl group at its side chain is prepared through copolymerization with an α-olefin having a hydroxyl group and used for the melt-forming (see, for example, JP-A-2001-302868). However, the PVA resin suffers from abnormal fluidization in the melt-forming process, thereby requiring further improvement in formability.
It is therefore an object of the present invention to provide a PVA resin composition which is excellent in melt-formability and free from emanation of odor in the melt-forming process, and is capable of providing a product free from residual odor, a highly transparent film and a laminate film having higher interlayer adhesive strength.